It is well known that the esterification of acid chlorides and alcohols can be carried out in the presence of a base, such as sodium or potassium hydroxide, in aqueous solution. Even when a two-phase system is necessary due to solubility limitations, the reaction will proceed readily. However, when a thiol ester is desired and thus a mercaptan is used instead of an alcohol, the reaction does not proceed under the same conditions. Different and less effective anhydrous reaction conditions have been used. For example, U.S. Pat. No. 4,692,184 (Lee, Sept. 8, 1987) discloses a method of making pyridine carbothioates in Examples 141and 146-149, but the only yield reported is 17.21%. The pyridine carboxyl chloride was mixed with the desired alkyl thiol and tetrahydrofuran in the presence of potassium tert-butoxide, and the resulting pyridine carbothioate was isolated. Therefore, there remains a need in the art for an improved process having improved yields, and preferably requiring less expensive solvents and/or less expensive bases.